1. Field of the Invention
The present invention relates to a solid-state image sensor and an image sensing system.
2. Description of the Related Art
In the observation of a biological body, it is necessary to monitor an image by light in a wavelength range on a short-wavelength side (to be referred to as the first wavelength range hereinafter) and an image by light in a wavelength range on a longer-wavelength side (to be referred to as the second wavelength range) than the first wavelength range at the same time. A method of performing image capturing using an image sensor for the first wavelength range and an image sensor for the second wavelength range, and superimposing the thus obtained images on each other is plausible. However, it is difficult to reduce the size and cost. It is, therefore, necessary to obtain images in two different wavelength ranges using a single image sensor.
Japanese Patent Laid-Open No. 2008-005213 discloses a CCD solid-state image sensor having a light receiving portion for a wavelength range on a short-wavelength side (to be referred to as the first light receiving portion) and a light receiving portion for a wavelength range on a long-wavelength side (to be referred to as the second light receiving portion). In this CCD solid-state image sensor, first, exposure of the first light receiving portion and the second light receiving portion (signal charge accumulation) is started at the same time. Next, after a generation portion generates a pulse, the signal charge of the second light receiving portion is swept in response to an LOD pulse, and the exposure of the second light receiving portion by a pulse reflected from an object is started. Then, all the signal charges of the first light receiving portion and the second light receiving portion are read out at once in response to a readout pulse. That is, in the solid-state image sensor described in Japanese Patent Laid-Open No. 2008-005213, the exposure period (signal charge accumulation period) of the light receiving portion for the wavelength range on the long-wavelength side is shorter than that on the short-wavelength side.
A conventional technique has relatively different sensitivities to light beams in different wavelength ranges. The CCD solid-state image sensor described in Japanese Patent Laid-Open No. 2008-005213 adds a dedicated overflow drain to independently change a pixel accumulation period. Therefore, the area of the photoelectric converter of a pixel to detect light in the wavelength range on the long-wavelength side is reduced, decreasing a sensitivity to long-wavelength light.
Furthermore, in the solid-state image sensor described in Japanese Patent Laid-Open No. 2008-005213, since the exposure period (signal charge accumulation period) of the light receiving portion for the wavelength range on the long-wavelength side is short, the detection of the long-wavelength light can be insufficient.